Chlorine (Cl). Diagram of the nuclear composition and electron configuration of an atom of chlorine-35 (atomic number: 17), the most common isotope of this element. The nucleus consists of 17 protons (red) and 18 neutrons (yellow). 17 electrons (white) bind to the nucleus, successively occupying available electron shells (rings). The stability of an element's outer electrons determines its chemical and physical properties. Vital to life in ionic form, chlorine is a halogen in group 17, period 3, and the p-block of the periodic table. In elemental form it is a yellow-green, reactive, toxic gas (Cl2) that liquefies at minus 34 degrees Celsius.